1. Field of the Invention
The present invention relates to photomasks for near-field light exposure, methods for making patterns using the photomasks, and apparatuses for making patterns including the photomasks.
2. Description of the Related Art
Higher resolution in photolithography is essential for progress in large-capacity semiconductor memories and high-speed, highly integrated CPUs. In general, the fine-patterning limits of photolithographic apparatuses are approximately the wavelengths of light rays emerging from light sources. Thus, near ultraviolet lasers are used as light sources of the photolithographic apparatuses to achieve fine lithographic processing with resolution on the order of 0.1 mm due to shorter wavelengths. For achieving a higher resolution of less than 0.1 mm, however, photolithography still has some unsolved problems, such as development of light sources that can emit shorter-wavelength light and development of lenses that can be used in this wavelength region.
Another type of micromachining apparatus for achieving a resolution of less than 0.1 mm includes a near-field optical microscope. In this apparatus, for example, a resist is exposed to evanescent light oozing out from a micro-aperture less than 100 nm at a region that is smaller than the light wavelength limit. The lithographic apparatus including the near-field optical microscope has the disadvantage of low throughput because micromachining is performed with one or several processing probes with a single stroke.
A countermeasure for improving the throughput is disclosed in Japanese Patent Laid-Open No. 8-179493. In this method, a prism is provided for a photomask. Light is incident on the prism under the total reflection condition, and the photomask pattern is transferred to the resist at a time using evanescent light oozing out from the total reflection surface. In the exposure apparatus using the evanescent light, however, the thickness of a shading film of the mask must be as thin as possible.
Another near-field light exposure mask is disclosed in Japanese Patent Laid-Open No. 2001-5168. This patent publication points out that the thickness of the shading film of the mask must be several tens of nm, and that the upper limit of the thickness of the shading film provided on a flat mask substrate is 50 nm. Thus, a substrate of the mask disclosed in this patent publication has grooves on which shading films are formed.
However, these known technologies do not disclose or suggest the dependence of the light intensity right below the aperture on the thickness of the shading film and the width of the aperture formed in the shading film, although they suggest some thickness information on the shading film of the photomask for near-field light exposure, as described above.
When a complicated pattern of fine apertures having different widths is provided in the mask, the near-field intensity varies with the width of each aperture of the mask. Thus, it is difficult to find a condition for exposing different positions through the apertures with the same light exposure. Even if the condition is found, the tolerance thereof is narrow.
Furthermore, the near-field of the fine aperture is affected by the aperture width; hence, a variation in width of fine apertures causes a change in size of the resist pattern formed by the exposure. Thus, such a variation must be suppressed. In addition, the near-field intensity would be lower than the intensity of the propagating light in the near-field mask exposure; hence, a mask structure that can utilize the near-field light with the maximum efficiency is required.